1. Field of the Invention
The present invention relates to a heating apparatus including a plurality of external heaters configured to heat an image heating rotational member that heats an image on a recording material.
2. Description of the Related Art
In recent years, it is desired by the market that an image forming apparatus, such as a copying machine, a printer, or an multifunction peripheral (MFP), has a high processing speed, is capable of printing a high quality image and executing color printing, and can save energy. It is also desired that an image forming apparatus is capable of executing printing on various type of recording media, such as thick paper, rough paper, rugged paper, and coated paper, and has a high productivity (i.e., is capable of printing a large number of print sheets in a unit time).
Under such circumstances, in an electrophotographic image forming apparatus, it is necessary to increase the heating property of a heating apparatus in order to increase the productivity particularly when a recording material having a large grammage is used.
However, the amount of heat necessary to fix a recording material having a large grammage (thick paper) is far larger than that necessary to fix a recording material having a small grammage (thin paper). Therefore, a large amount of heat is lost from a fixing roller (image heating rotational member) at the time of fixing. Accordingly, the surface temperature of the fixing roller may decrease and fixing failure may occur. Accordingly, in a case of fixing thick paper, in order to secure the fixing property (the bonding strength between a toner and a recording material), a conventional method executes fixing processing by feeding a recording material into a heating apparatus at a relatively low speed.
If a fixing roller is used that includes a pipe-shaped metal core on which a heatproof elastic layer made of a material such as silicon rubber or fluorine rubber is formed, the above-described decrease of the surface temperature of a fixing roller may occur partly due to the low thermoconductivity of the metal core and the elastic layer. More specifically, in this case, the heat of a heat generation member (a halogen heater, for example), which is provided in the core of the fixing roller, is shielded by the core and the elastic layer. Thus, the heat of the heat generation member is not appropriately applied on the surface of the fixing roller.
In this regard, a conventional method employs a fixing roller including no such elastic layer. In this case, the decrease of the surface temperature of the fixing roller becomes small because no elastic layer is used. However, because of a thick core used in this case, the surface temperature of the fixing roller may decrease, which may shield the heat as described above.
Furthermore, if a core including no elastic layer is used, in recording on a recording material having a considerable rug on its surface, a toner applied on a concave portion of the surface of the recording material and the fixing roller may not appropriately contact each other. Thus, the toner on the concave portion may not be normally fixed.
Furthermore, in developing a color image, the surface of the image cannot be evenly fused. Accordingly, in this case, phenomena of unevenly fixed toner, uneven gloss, and uneven color may occur. Therefore, the image quality may degrade.
Accordingly, it is useful to provide a fixing roller with such an elastic layer in order to enable recording on various types of recording materials and increase the image quality. On the other hand, if a fixing roller is rapidly heated with a heat generation member having a high normal rated power in order to prevent the decrease of the surface temperature of the fixing roller, the temperature of the core may rapidly rise. In this case, a bonding layer between the core and the elastic layer may be damaged or broken due to thermal degradation. As a result, the elastic layer may break away from the core or the elastic layer may be damaged or broken due to softening deterioration or hardening deterioration caused by the heat.
Consequently, Japanese Patent Application Laid-Open No. 2002-251096 discusses a method for executing fixing without reducing the speed of feeding a recording material through a heating apparatus. In this method, a fixing roller is heated from its external surface by an external heating roller that contacts the outer surface of the fixing roller. The conventional method can prevent the decrease in the surface temperature of a fixing roller while preventing the rise in the temperature of the core.
Furthermore, it is useful to set the temperature of an external heating roller at a high value in order to increase the heating property of the external heating roller. However, the temperature of the external heating roller cannot be set at a very high value considering the limit of heat resistance of the external heating roller. On the other hand, if a wide contact area between an external heating roller and a fixing roller is secured, the temperature can be set low. However, in this case, the size of the external heating member itself may become large and thus the size of the heating apparatus may become large. Accordingly, Japanese Patent Application Laid-Open No. 2004-37555 discusses a relatively small size heating apparatus including a plurality of external heating members and capable of increasing the heating property of an external heating roller.
Meanwhile, as a method for adjusting the temperature of a heating roller, a conventional method discussed in Japanese Patent Application Laid-Open No. 08-185080 powers on and off a heat generation member provided in a heating roller.
However, when a plurality of external heating rollers is used, the amount of heat transferred from a downstream external heating roller to a fixing roller may decrease. As a result, when the temperature adjustment is executed by powering on and off a heat generation member as discussed in Japanese Patent Application Laid-Open No. 08-185080, the length of time of supplying power to the heat generation member provided in a downstream external heating member is short. Accordingly, the surface temperature of the fixing roller may become uneven. The problem like this will be described in detail below.
Considering the capacity of an external heating roller that heat a fixing roller when a plurality of external heating members is provided, it is useful to set a target temperature in adjusting the temperature of each external heating roller at a high value. In this case, the target temperatures in adjusting the temperature of a plurality of external heating rollers become substantially the same.
Meanwhile, when the heat is transferred from a fixing roller to a recording material, the temperature of an area of the fixing roller heated by an upstream external heating roller may decrease. Thus, the difference between the temperature of the upstream external heating member and that of the fixing roller becomes large. Therefore, a large amount of heat is transferred from the upstream external heating roller to the fixing roller and the time of supplying power to the heat generation member of the upstream external heating roller may become longer.
On the other hand, the area of the fixing roller heated by the downstream external heating roller is heated by the upstream external heating roller. Accordingly, the difference between the temperature of the downstream external heating roller and that of the fixing roller is small. As a result, a small amount of heat is transferred from the downstream external heating roller to the fixing roller. Therefore, the time of supplying power to the heat generation member becomes short. In this case, the downstream heating roller is heated for a shorter period of time. Accordingly, the surface temperature of the fixing roller may become uneven.